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In a dichotic paradigm, duration-deviant mismatch negativity (MMN) largely reflects standard stimuli presented to the same ear as the deviant, suggesting an independent representation for each ear. We sought to assess this representation independence. Twenty-two participants, presented left (all 2000 Hz, 70-dB sound pressure level [SPL]) and right (all 800 Hz, 85-dB SPL) ear sounds, attended to 1 ear per block. A series of successive trains of 6–30 sounds were presented to each ear. Although 1 ear received short (40 ms) standard sounds, the other received an equal number of long (120 ms) standards. In the next train, the standard durations were switched between the ears. Stimulus onset asynchrony ranged from 120 to 440 ms. Duration deviants (240 ms) replaced the final standards of some trains. MMN latency simultaneously moved earlier for the ear changing from long to short standards and later for the ear changing from short to long standards. Based on a simple linear model, more than 80% of the memory trace reflected within-channel standards. We conclude that independently changeable memory traces underlie dichotic presentation. Separation of the representation for the attended subset of stimuli is proposed as core to the mechanism of sustained attention.